This year’s Nobel Prize in Physics is all about neutrinos, perhaps the most puzzling particles in the universe. Like ghosts, they pass through the thickest walls – indeed, they penetrate straight through the entire globe. They have no electric charge, race at almost the speed of light and weigh almost nothing at all – for a long time, we actually thought they were massless! Studying neutrinos is a real challenge!

This year’s Laureates, Takaaki Kajita and Arthur B. McDonald, led two large research groups which discovered that these remarkable particles are even more puzzling than we imagined. They behave like chameleons: while travelling through space, they are continuously switching between different identities.

Neutrinos are everywhere. They are constantly created – inside the Sun, when the “cosmic rain” from space collides with the atmosphere, through the decay of atomic nuclei in the Earth’s crust – and even in our muscles.

Every second, billions of neutrinos pass through our bodies, unseen and unnoticed. These elusive particles rarely interact and are almost impossible to capture. Yet their ghostlike nature seems very useful: while the light generated in the centre of the Sun never reaches the Earth, neutrino radiation from the fusion processes in the solar core passes straight out. Measuring the number of neutrinos from the Sun therefore provides a means of measuring the temperature in its core. The first experiment aimed at recording solar neutrino radiation was initiated fifty years ago and was followed by many more. It soon turned out – to everyone’s surprise – that up to two thirds of the expected number of neutrinos was missing. The neutrinos seemed to be disappearing along the way!

One theory, among many attempts to explain this remarkable phenomenon, was that during its journey a neutrino could change its identity and be transformed into another type of neutrino that was not visible in the detector. This is because there are three types of neutrinos, of which the Sun only produces one – and the experiments were designed to detect only this one type.

But can a particle actually change identity? This mystery was solved with the help of two enormous underground detectors.

The Super-Kamiokande detector was located in a zinc mine in Japan, 1,000 metres below the Earth’s surface, and contained 50,000 tonnes of water – more than enough to fill all the bathtubs in Stockholm! This detector recorded the neutrinos created when cosmic radiation collides with the Earth’s atmosphere. To their great surprise, scientists discovered that many fewer neutrinos reached the detector from below – that is, from the other side of the globe – than from the atmosphere above. The neutrino types that are created in the atmosphere also seemed to be disappearing along the way!

Neutrinos pass unimpeded through the Earth. So Kajita-san and his team realised that the great distance between the production point on the other side of the Earth and the detector gave the neutrinos extra time to change identity. This finding was presented around the turn of the millennium.

Around the same time, on the other side of the Earth, a huge detector was built for observation of the neutrinos that are produced in the solar core. Unlike earlier detectors, the Sudbury Neutrino Observatory in Canada – two kilometres below the Earth’s surface – could effectively record all three types of neutrinos. McDonald and his research team were then able to show that the total number of neutrinos from the Sun reaching the Earth corresponded closely with expectations, but that the type of neutrinos produced in the solar core was too few. They did not seem to have disappeared, though; instead, they must have been transformed into another type of neutrino and switched identity.

The results of both these experiments have one explanation in common: quantum theory describes particles travelling through space as waves. If the three types of neutrinos have different masses, they are described by waves with different frequencies. The interaction between these waves while they travel through space underlies the neutrinos’ metamorphosis. The phenomenon is called neutrino oscillations and can only occur if neutrinos have mass.

The number of neutrinos in the universe is enormous. The discovery that they are not massless, as we believed for a long time, is therefore of crucial importance to our understanding of the structure of the universe – and also has far-reaching consequences for cosmology.

Professor McDonald, Professor Kajita: You have been awarded the Nobel Prize in Physics for the discovery of neutrino oscillations, which shows that neutrinos have mass. On behalf of the Royal Swedish Academy of Sciences it is my honour and great pleasure to convey to you the warmest congratulations. I now ask you to step forward to receive your Nobel Prizes from the hands of His Majesty the King.

Learn more

This year 12 new laureates have been awarded for achievements that have conferred the greatest benefit to humankind.

Their work and discoveries range from cancer therapy and laser physics to developing proteins that can solve humankind’s chemical problems. The work of the 2018 Nobel Laureates also included combating war crimes, as well as integrating innovation and climate with economic growth. Find out more.

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